HONG KONG (Reuters) - Hong Kong warned people with heart or lung problems to avoid outdoor activities on Friday as the territory experienced one of its most polluted days of the year, with the hills across the harbor almost invisible.

Pollution monitoring stations registered "very high" readings in several spots around the former British colony, and the Environmental Protection Department said the poor air was expected to continue.

Hong Kong's air has become increasingly clogged with pollutants from cars, ships, power plants and a booming manufacturing sector across the border in China's Guangdong province.

Air Pollution Index (API) readings surpassed 101, entering what the EPD considers "very high," at several sites, including the Central business district, which hit 150 by mid-afternoon, and the Causeway Bay shopping area, which hit 127.

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The figures were among the highest this year, a spokesman for the EPD said. Readings are typically worse during winter months.

"We expect that the regional background air pollution will continue to be high and that air dispersion will remain poor tomorrow. The general and roadside API readings are expected to remain at higher than normal levels," the department said in a written statement.

With very high readings, the government recommended "people with heart or respiratory illnesses should reduce physical exertion and outdoor activities, and avoid prolonged stay in roads and streets with heavy traffic."

(Reporting by John Ruwitch; Editing by Nick Macfie and Alex Richardson)